1. Field of the Invention
This invention relates to diesel and middle distillate fractions excluding jet and to additives used for improving the cetane number of such fractions.
2. Description of Related Art
Fuel ignition in diesel engines is achieved through the heat generated by air compression, as a piston in the cylinder moves to reduce the cylinder volume during the compression stroke. In the engine, the air is first compressed, then the fuel is injected into the cylinder; as the fuel contacts the heated air, it vaporizes and finally begins to burn as the self-ignition temperature is reached. Additional fuel is injected during the compression stroke and the fuel burns almost instantaneously, once the initial flame has been established. Thus, a period of time elapses between the beginning of fuel injection and the appearance of a flame in the cylinder. This period is commonly called "ignition delay" and must be relatively short in order to avoid "diesel knock". A major contributing factor to diesel fuel performance and the avoidance of "diesel knock" is the cetane number of the diesel fuel. Diesel fuels of higher cetane number exhibit a shorter ignition delay than do diesel fuels of a lower cetane number. Therefore, higher cetane number diesel fuels are desirable to avoid diesel knock. Most diesel fuels possess cetane numbers in the range of about 40 to 55 and a sulfur content of about 500 ppm and less. A good correlation between ignition delay and cetane number has been reported in "How Do Diesel Fuel Ignition Improvers Work" Clothier, et al., Chem. Soc. Rev, 1993, pg. 101-108 in the region 3.ltoreq.t igni.ltoreq.8 m sec using the equation CN=91-6.4 t igni, which reflects contributions by engine timing and levels of additives in the fuel. Correcting the equation to remove the influences of timing and additives results in the equation CN=8.5-6.0 t igni, which formula was used to calculate the cetane indices reported in this specification.
Cetane improvers have been used for many years to improve the ignition quality of diesel fuels. The use of cetane improvers is increasing due to the increased demand for diesel fuel which has resulted in a widening of the fraction recovered, the so called middle distillate fraction, and the lower natural cetane number of diesel base stocks caused by more severe refining of crude oil and the effort made to produce low emission diesel.
Many types of additives have been prepared and evaluated to raise the cetane number of diesel fuel. Such additives include peroxides, nitrates, nitrites, azo compounds and the like.
Alkyl nitrates such as amyl nitrate, hexyl nitrate and mixed octyl nitrates have been used commercially with good results. Other nitrates such as 2-methyl-2-nitropropyl nitrate (U.S. Pat. No. 4,536,190) have been suggested as cetane improvers but found shock sensitive. However, it is generally accepted that organic nitrates, more specifically the commercial 2-ethylhexyl nitrate, are the most cost-effective additives to improve cetane number of diesels. Because of its relatively low cost, and environmentally friendly nature (ashless), there has been limited work done in this area to replace the 2-ethylhexyl nitrate.
U.S. Pat. No. 4,992,605 discloses a process for producing high cetane hydrocarbons in the diesel boiling range, by hydroprocessing tall oil or vegetable oils such as canola, sunflower, soybean and rapeseed oil at temperatures in the range from 350.degree. C. to 450.degree. C. and pressures of 4.8 to 15.2 MPa. The hydrocarbons mixture produced by this process has a relatively high cetane number (50-85 CN), however, relatively high levels (10-15%) are required to increase the cetane number of the diesel fuel by about 3 to 5 cetane numbers. Moreover, because of the waxy nature of the material, it has relatively high cloud point (4.degree.-16.degree. C.) which limits its usefulness to blending into summer diesel.
U.S. Pat. No. 4,585,461 refers to a method of manufacturing a cetane improver fusel oil, a waste product from the distillation of alcoholic beverages. Fusel oil provides a cheap source of ethyl alcohol (5 to 25%), isobutyl alcohol (16 to 33%) and isoamyl alcohol (30 to 77%). However, it is mentioned that fusel oil is foul smelling, quite toxic and one of the alcohol is a teratogen. Moreover, lower molecular weight nitrates such as ethyl nitrate or amyl nitrate, tend to be explosive in inverse proportion to their molecular weight. Such materials are hazardous if their molecular weight is 76, but decreasingly as their weight reaches 174. "Fusel" nitrate has a molecular weight of 119 and is moderately hazardous.
Organic nitrates and organic peroxides are well known to cause substantial increases in cetane number of diesel fuels. It is generally accepted that organic nitrates, more specifically the commercial 2-ethylhexyl nitrate (DII-3 sold by Ethyl Petroleum Additives), are the most cost-effective additives to improve the cetane number of diesel fuels.
Clothier et al in "How Do Diesel Fuel Ignition Improvers Work" Chem. Soc. Rev. 1993, pg 101-108 have reported that the ignition delay using half-and-half mixture of 2-ethylhexyl nitrate and di-tert-butyl peroxide was not better than using either additive on its own. Similar results to those reported by Clothier have been obtained with tert-butyl perbenzoate and 2-ethylhexyl nitrate in Example 2 herein.
G.B. Patent 2,227,752A teaches that cetane number of a hydrocarbon-based fuel is increased by the addition of a minor amount of a parketal of the formula R.sub.2 R.sub.3 C(OOR.sub.1).sub.2 wherein R.sub.1 is a C.sub.4 -C.sub.10 tertiary alkyl group and R.sub.2 and R.sub.3 together with the attached C atom form a cycloalkane ring optionally substituted by one or more C.sub.1 -C.sub.4 alkyl radicals or other essentially inert substituents. The perketal is not used in combination with an alkyl nitrate.
More recently EP0537931 discloses a fuel composition for reducing emissions on combustion consisting of a middle distillate fuel, organic nitrate combustion improver and a tert-alkyl peroxyalkanoate or peroxybenzoate.
U.S. Pat. No. 5,114,433 describes a process for improving the cetane number of a directly distilled diesel fuel by intimately contacting same with hydrogen peroxide in the presence of carboxylic acid or with a percarboxylic acid in the presence or absence of hydrogen peroxide.
G.B. Patent 2,227,751A discloses a hydrocarbon-based fuel to which has been added a minor amount, sufficient to increase the cetane value of the fuel, of a perester of the formula R.sub.1 COOOR.sub.2 where R.sub.1 is a C.sub.5 -C.sub.20 secondary or tertiary alkyl group and R.sub.2 is a C.sub.4 -C.sub.10 tertiary alkyl group.
U.S. Pat. No. 4,365,973 discloses a middle distillate fuel additive composition to improve cold flow properties, cetane, pour point, wax formation and anti-icing characteristics and comprising a cold flow improver, preferably vinyl acetate-ethylene copolymer, a cetane improver comprising paraffinic nitrate or a mixture of nitrates and an anti-icer comprising an aliphatic alcohol or cyclic aliphatic alcohol having from 1 to 6 carbon atoms.
EP 467,628 discloses a middle distillate composition to reduce atmospheric pollutants (NOx, CO and/or hydrocarbons). The fuels incorporate a peroxy ester combustion improver of the formula (R--O--O--(CO)).sub.n R.sup.1 where R and R.sup.1 are both hydrocarbyl groups. Suitable peroxy esters include tert-butyl peroxydodecanoate, di-(tert-butyl-diperoxy) phthalate and 1,1-dimethylpropylperoxy benzoate. The peroxy ester is used in combination with an organic nitrate ester such as 2-ethylhexyl nitrate.
U.S. Pat. No. 4,330,304 discloses a fuel additive for improving the combustion efficiency of fuels for diesel engine, jet engines, boiler and other apparatus. The additive comprises a hydroperoxide such as cumene hydroperoxide, a nitroparaffin and propylene oxide.
EP 293,069 discloses a cetane improver comprising tetralin hydroperoxide. The cetane improver is produced by partially hydrogenating a naphthalene or alkyl naphthalene-containing hydrocarbon oil to obtain tetralins, which are then partially oxidized to produce a hydrocarbon oil containing tetraline hydroperoxides.
U.S. Pat. No. 5,258,049 discloses a diesel fuel containing the nitric acid ester of 1-phenyl ethanol as cetane improver.